Energetic evaluation of Ba– and Sr–Si clathrate formation at high pressures by first-principle pseudopotential calculations

Energetic evaluations of BaSi2, Ba8Si46, BaSi6, SrSi2, Sr8Si46, and SrSi6 at 0 to 15 GPa (or 20 GPa) have been performed using first-principle calculations based on the density-functional theory. From the calculations, Ba8Si46 is expected to be present as an equilibrium phases at 10 GPa and 15 GPa, while BaSi6 is considered to be formed at 15 GPa and 20 GPa. SrSi6 is expected to be present at 10 GPa and 15 GPa. Sr8Si46 would not be present at any pressure from 0 to 15 GPa, since SrSi6 + SrSi2 is more stable at pressures of 10 GPa and 15 GPa than Sr8Si46, while SrSi2 + Si is more stable at 0 GPa and 5 GPa. This is consistent with the observed absence of Sr8Si46 in contrast to the presence of Ba8Si46. In addition, the phase transformation of disilicides by pressurization could almost be properly predicted, except for the observed higher stability of the cubic phase of BaSi2 compared to its trigonal phase at lower temperatures and that of the cubic phase of SrSi2 compared to its tetragonal phase at ambient pressure.